1. Field of the Invention
The present invention relates to a touch panel device to be used as a display/input device for an electronic device such as a car navigation apparatus.
2. Description of the Related Art
In recent times, various electronic devices, such as an ATM and a car navigation apparatus, are equipped with a panel device which can permit a panel (or screen) to be used for data/information display and data/information entry. This type of panel device is usually referred to as “touch panel device.” Use of the touch panel device is particularly advantageous to a car navigation apparatus because a large amount of data/information should be displayed and a large amount of data/information should be input into the display panel but the size of the display panel screen is relatively small. Here, the term “data/information” includes any numbers, letters, images, signs and moving picture which can be displayed on the screen.
In general, a touch panel includes a display layer and a touch sensor layer laminated over the display layer. The touch sensor layer includes transparent resistance layers and electrodes. The display layer is, for example, an LCD or an organic electroluminescent (EL) display. Data/information entry buttons or icons (referred to as “operation buttons”) are displayed on the display layer. An area used to display each data/information entry button is called a “display area.” Thus, a plurality of display areas are usually defined on the display layer. If the touch sensor layer is a pressure-sensitive resistance-embedded layer (resistance film type), the touch sensor layer includes two transparent resistance layers facing each other. Each transparent resistance layer is made of a glass sheet or plastic film on which an ITO (Indium Tin Oxide) is deposited.
The principle of operation of the touch panel device will be described briefly. When a user presses one of the operation buttons displayed on the display layer via the touch sensor layer with his finger, a pressure applied on the button changes an electric resistance in the touch sensor layer at the position corresponding to the pressed button. A controller of the touch panel device detects the pressed position (touched area) on the touch sensor layer when the resistance changes in the touch sensor layer. The controller then determines the relationship between the touched position and the location of the pressed button so as to determine which button is pressed.
FIG. 1 of the accompanying drawings illustrates a configuration of a display screen of a conventional touch panel device used in a car navigation apparatus. In this drawing, reference numeral 10 designates the display screen of the touch panel device. Each solid-line rectangle 11 designates a display area on the display layer of the touch panel device. The display areas 11 show selectable operations (menu) of the car navigation apparatus. Each broken-line rectangle 12 designates a touch area on the touch sensor layer. The touch area 12 is larger than the display area 11. The touch area 12 concentrically overlaps the corresponding display area 11 corresponding to the display area of the display layer. When a user presses a certain touch area 12 with his finger, the controller of the touch panel device determines that the operation button displayed in the corresponding display area 11 is pressed. The solid line rectangle for the display area 11 is visible to the user, but the broken line rectangle for the touch area 12 is not visible.
The position and size of each of the display areas 11 and touch areas 12 of the touch panel device can be arbitrarily determined by software control of the touch panel device. The touch panel device can present a plurality of choice or menu screens on the single display screen. The user can touch and select a desired item from a menu of items.
The position and the size of the display area 11 may be identical to those of the corresponding touch area 12. Generally, however, the touch area 12 is larger than the display area 12, as shown in FIG. 1. This is to ensure a touching operation made by a user is properly sensed by the touch panel device even if the user touches slightly outside the display area 11.
In practice, when a user intends to press the touch area 12 on the touch panel 10 with his finger, the user's finger sometimes touches not only the touch area 12 but also another area of the display screen, because fingers have various shapes, nails sometimes protrude from the fingers, and the approach direction of the finger to the touch panel affects the contact between the finger and the touch panel. As shown in FIG. 2 of the accompanying drawings, for example, if a user's finger 13 having a relatively long nail 14 presses the point A in the display area 11, the user may also unintentionally touch the point B on the touch panel with his nail 14 at the same time.
If the touch panel device is a pressure-sensitive resistance-embedded panel, the controller of the touch panel device generally utilizes an algorithm to determine a middle point of two points simultaneously touched on the panel screen 10. In case of FIG. 2, the controller of the touch panel device considers that the middle point C between the points A and B is pressed if the points A and B are pressed at the same time. As seen in FIG. 2, the point C is outside the touch area 12. The periphery of the touch area 12 defines an outer boundary of the effective touching range. Thus, a desired operation is not triggered although the user believes that the user has pressed the intended display area 11. In fact, the finger 13 presses the point A in the touch area 12, but the touch panel device does not operate.
To prevent such inconveniences, each touch area 12 may be simply expanded. However, if the larger touch areas 12 are used, the number of menu boxes 11 is reduced. In addition, the spacing between the adjacent touch areas 12 becomes small, so that the user may touch an unintended touch area 12, i.e., the possibility of incorrect operations increases.